1. Technical Field
The present disclosure relates to an image sensor, and more particularly, to an image sensor that is capable of producing a stable reference signal.
2. Discussion of the Related Art
Image sensors convert optical information into an electrical signal. Image sensors are used in connection with, for example, digital cameras, camcorders, personal computers (PCs), game devices, security cameras, micro-cameras for medical use, and robots.
An image sensor includes an active pixel region and an optical black region. In the active pixel region, a plurality of unit pixels are arranged in a matrix pattern. The unit pixels convert incident light into an image signal. The optical black region is formed adjacent to the active pixel region. The optical black region provides a constant reference signal regardless of the incident light, and includes a plurality of shaded unit pixels. In particular, the optical black region prevents a level of the image signal from varying according to temperature changes. For example assuming that a voltage level of the reference signal is related to ambient temperature, a difference between a voltage level of the image signal and that of the reference signal is calculated using a signal generated by the incident light.
However, when high intensity light is incident on the active pixel region, in particular, around the periphery of the optical black region, excess electrons generated by a photoelectric converter of a unit pixel overflow to the shaded unit pixels in the optical black region, thereby distorting the reference signal. When the distorted reference signal is used, the difference between the voltage level of the image signal and that of the reference signal becomes smaller than when a normal reference signal is used. As a result, image quality is reduced.